1. Field of the Invention
This invention relates to high current electrical switching apparatus for electrical power distribution systems. More particularly, it relates to the joining of the multiple switching units together side-by-side, and electrically connecting them to share the current load.
2. Background Information
Electrical switching apparatus for electric power distribution systems includes circuit breakers and network protectors which provide protection, and electrical switches for isolating parts of the distribution system and for transferring between alternative sources. While families of such switches are produced having a range of current ratings, some applications require higher current ratings than are available from the standard units. It is not practical to make a dedicated switch for such applications in view of the limited demand. It is therefore common to mount a pair of such switches side-by-side and connect the poles to share the current. This parallel construction technique is of particular value with molded case switches where the required investment in the molded case is quite large and can be avoided by adjoining two smaller cases. Typically, the casings have been bolted together. U.S. Pat. No. 4,884,047 suggests joining the molded cases together by bolting a spacer to the two casings and then filling the remaining space with a cold-cast resin which hardens and engages reliefs in the confronting sidewalls of the molded casings.
When joining two multipole switches side-by-side, it is standard practice to parallel connect adjacent pole units, either internal to the switch or immediately outside. This produces an AABBCC phase identity. This method of parallel connecting adjacent poles is quite simple; however, it has several disadvantages. Not only is the switch dedicated to a single three pole construction, but the parts used to perform the parallel connection function are used exclusively in that larger frame size and thus in limited production volumes. More importantly, we have found that the ampacity of the individual pole units are not additive when adjacent poles are parallel connected. In fact, in larger sizes, such as 2,000 or 3,000 ampere pole units, the double-pole construction must be derated by as much as twenty percent compared to the combined reading of the two individual pole units.
There is a need, therefore, for improved electrical switching apparatus which combines two smaller multipole switching units to produce a unit with a higher current capacity.
There is a more particular need for a simple, reliable arrangement for joining multipole switching units, especially which does not require cold-casting a resin.
There is also a need for such multipole switching apparatus combining two smaller units which does not result in a substantial derating of the unit over the combined readings of the individual units.